In dual-fuel gas turbines, the turbine operates by burning either a gaseous fuel or a liquid fuel, the latter fuel being typically distillate oil. Whenever burning one fuel, the nozzles for the other fuel must be continuously purged. For example, when gaseous fuel is burned in the turbine combustors, the liquid fuel, atomizing air and water injection nozzles of the combustors are purged using cooled compressor discharge air. When burning liquid fuel, atomizing air is supplied to the combustor for atomizing the liquid fuel, while the gas fuel nozzles are purged using compressor discharge air directly from the casing. In an air atomizing mode during liquid fuel operations, compressor discharge air at a reduced temperature, for example, of 225.degree. F., is supplied to atomize the liquid fuel flowing through the liquid fuel nozzle, while during a purge mode, the compressor discharge air at reduced temperature and pressure is supplied as purge air to the liquid fuel, atomizing air and water injection nozzles. It will be appreciated that during the atomizing air mode for liquid fuel operations, the pressure ratio is higher, i.e., a higher pressure ratio is needed to atomize the fuel oil than simply to keep the nozzle passages clear. Thus, in the purge mode during gas fuel operations, the flow of compressor discharge purge air is reduced, for example, to about half the flow than would be the case in the air atomizing mode but at the same temperature.
As well known, a gas turbine compressor discharges air at a substantially constant temperature, for example, on the order of 800.degree. F. In the atomizing air mode, 100% of the system air is used in the combustors to atomize the liquid fuel, while in the purge mode, approximately only 50% of the system air is used for purging. In both the atomizing and purge air modes, the compressor discharge air is reduced in temperature, i.e., cooled in a heat exchanger, to approximately 225.degree. F. A cooling medium, typically, and hereafter referred to as water, is placed in heat exchange relation with the compressor discharge air to reduce the air temperature to the desired temperature. This reduced air temperature is obtained by controlling the flow of the water through the heat exchanger. The cooling water inlet temperature to the heat exchanger varies considerably based on seasonal ambient conditions. The system must also be sized for worst-case conditions, i.e., the atomizing air condition since in the purge mode, the heat rejection to the cooling water in the heat exchanger is approximately half that of the heat rejection in the atomizing air mode.
In a conventional system for controlling the temperature of the compressor discharge air, the flow of cooling medium through the heat exchanger is controlled by a temperature regulating valve responsive to the temperature of the compressor discharge air exiting the heat exchanger. Should the ambient temperature of the cooling water, however, be low, for example, when using cooling water supplied during winter conditions, the reduction in water flow can cause the water to boil in the heat exchanger. This, in turn, can result in damage to the heat exchanger and cause a shutdown of the turbine. Recognizing that problem, an orifice bypass around the water flow control valve has been previously provided. This ensures a protective minimum flow of water through the heat exchanger when the magnitude of the heat rejected to the cooling water in the heat exchanger is such that the temperature of the air exiting the heat exchanger is lowered to a temperature causing the temperature control valve to close. Over-cooling the discharge air disadvantageously produces increased condensation which must be eliminated and otherwise compromises the system.
The problem of not being able to reduce the water flow sufficiently at lower ambient water inlet temperatures to the heat exchanger is exacerbated when the system is operated in the purge mode rather than in the atomizing air mode. In the purge mode, the heat rejected to the cooling water in the heat exchanger is greatly reduced, e.g., on the order of one-half, relative to the heat rejection in the atomizing air mode, and the required cooling water flow is thus reduced. Because the heat exchanger is sized for the maximum heat rejection at maximum cooling water temperature, the desired compressor discharge air temperature of 225.degree. F. becomes impossible to maintain at minimum heat rejection and low ambient cooling water inlet temperature conditions, resulting in over-cooling of the compressor discharge air.